Add custom format variables

[bczhc]

Seems I can't find a way to add custom format variables, for example, for the image exposure value (not the 'bias') calculated by the formula $$\log_2{(100N^2/(\rm{ISO}\cdot S))}$$, I could add a variable called $(EV_AT_ISO100) and use it in lighttable tooltip/overlay text formatting textbox. Or, can I modify the overlay text directly in lua scripts?

[wpferguson]

The overlay text is not accessible from Lua at the current time.

Lua does support almost all of the substitution variables supported by darktable, but it is a separate functionality contained in the Lua scripts string library and is not used by the darktable variable substitutions.